This invention relates to a flowmeter and more particularly to a magnetoinductive flowmeter for a medium, which is flowing through a measuring tube and has a minimum electrical conductivity, having at least two electric coils, which are arranged opposite one another on the measuring tube, for the purpose of inducing a magnetic field running essentially perpendicularly with respect to the longitudinal axis of the measuring tube, and having an electrode arrangement, which is likewise aligned essentially perpendicularly with respect to the longitudinal axis of the measuring tube and essentially perpendicularly with respect to the magnetic-field axis, for the purpose of measuring a measurement voltage induced as a result of the flow of medium.
Such a magnetoinductive flowmeter is preferably used as a flowmeter for liquids, slurries and pastes which have a specific minimum electrical conductivity. The basis for the measuring method is formed by Faraday's law of induction, which states that a voltage is induced in a conductor moving in a magnetic field. When using this law of nature in measurement engineering, the electrically conductive medium flows through a measuring tube, in which a magnetic field is induced perpendicularly with respect to the direction of flow. The voltage induced in the medium is tapped off by an electrode arrangement. Since the measurement voltage obtained in this manner is proportional to the average flow rate of the flowing medium, the volumetric flow of the medium can be determined from this, or else by taking into account the density of the mass flow.
EP 0 869 336 A2 describes a generic magnetoinductive flowmeter. The flowmeter comprises two opposing electric coils which induce the required magnetic field in the measuring tube perpendicularly with respect to the direction of flow. Within this magnetic field, each volume element, moving through the magnetic field, of the flowing medium with the field strength produced in this volume element contributes to the measurement voltage tapped off via measurement electrodes. The measurement voltage is fed on the input side to the downstream evaluation electronics. Signal amplification initially takes place within the evaluation electronics via an electronic differential amplifier, the differential amplifier operating with respect to a reference potential which generally corresponds to the earth potential. The evaluation electronics produces a value, on the basis of the measurement voltage, for the volumetric flow of the medium flowing through the measuring tube.
The electric coils for inducing the magnetic field running perpendicularly with respect to the longitudinal axis of the measuring tube are mounted on the outside of the measuring tube in a manner which is generally known. In order to protect the electric coils against damaging external influences—such as moisture, mechanical impact stress, etc.—they are usually extruded with plastic and are thus encapsulated.
One disadvantage here is the fact that, during manufacture, the electric coils together with the electrical lines and possibly the iron core need to be inserted in an injection-moulding die, and only then can injection moulding take place. If a plurality of individual parts are to be inserted in an injection-moulding die, it is necessary to ensure that their position with respect to one another does not change when filling the casting mould with the plastic. A precondition for inducing a precisely aligned magnetic field is for the coils to be positioned as precisely as possible with respect to the cast housing. All of these influencing factors entail correspondingly high manufacturing complexity for such an encapsulated electric coil arrangement.
It is therefore the object of the present invention to further improve a magnetoinductive flowmeter of the type described initially such that its electric coils can be produced in a manner which is simple in terms of manufacture and such that they are protected reliably against external influences.